This invention relates to a process for producing high-purity alkylene carbonates. More particularly, it relates to a distillation process in which alkylene carbonate distillates are produced, and in which a reduced amount of polyether polycarbonate impurities are formed during the distillation procedure.
Alkylene carbonates are well known materials in the chemical arts which are useful in a wide variety of end uses that include use as solvents, carriers, reactants, etc. owing to the favorable combination of physical and chemical properties they possess. The alkylene carbonates to be purified according to the present invention include those represented by the formula: 
which are the most commonly available alkylene carbonates, having R=hydrogen, methyl, or ethyl; and which correspond to ethylene carbonate, propylene carbonate, and butylene carbonate.
Alkylene carbonates may be produced by differing methods, with the most common commercially practiced method being that which is taught in U.S. Pat. No. 2,773,070 (incorporated herein by reference thereto) in which the catalyst is a tetraalkyl ammonium bromide. Depending upon the exact processing conditions used, the alkylene carbonate product may contain varying degrees of chemical impurities in addition to the desired alkylene carbonate. Thus, distillation is a commonly-employed technique used to purify the alkylene carbonate product.
The distillation of the alkylene carbonate according to the prior art is carried out in accordance with conventional distillation principles, involving a pot, condenser, and receiver. U.S. Pat. No. 6,156,160 (incorporated herein by reference thereto) describes one such method involving several distillation apparati connected to one another in a series arrangement. However, even when using the optimized configuration of equipment and processing conditions of this and other prior art methods, a substantial quantity of the contents of the bottoms in the distillation equipment comprises a polymerized form of the alkylene carbonate being purified, by virtue of its residence time in the distillation pot. The undesirable polymerized form of alkylene carbonate, which is technically considered to be a polyether carbonate exemplified by the structure: 
in which the individual R groups may be selected from the group: hydrogen, methyl, or ethyl, and where m and n may each be any integer between 1 and 5000 or greater. This material is believed to be formed by the self-condensation polymerization of the alkylene carbonate during distillation and de-carboxylation of the resulting polymer thus providing a polyethercarbonate impurity which is capable of reacting further yet with another molecule of the alkylene carbonate that is being purified. The result of the continuous auto-polymerization of the alkylene carbonate is that there is an increase in the average molecular weight of the material in the pot, which material includes both the alkylene carbonate being purified and the polymeric autocondensation product. The formation of the polyethercarbonate impurity from the alkylene carbonate being purified results in a reduction of the yield in the amount of desired alkylene carbonate distillate obtainable. Therefore, formation of the polyethercarbonates represents an undesirable side reaction in the distillation of alkylene carbonates because it reduces the total amount of final product obtainable as overheads. Thus, if a method of inhibiting formation of polyethercarbonates in the distillation pot during the distillation process were available, such method would result in increased product yields in the case of alkylene carbonate manufacture.
The present invention provides a distillation process for purifying alkylene carbonates which comprises the steps of first providing a distillation apparatus which includes a pot portion, a condenser portion, and a receiver portion. An alkylene carbonate is provided in the pot portion of the apparatus, along with an alkylene carbonate autopolymerization inhibiting substance as disclosed herein. An effective amount of heat is supplied to the pot portion to cause said alkylene carbonate to boil to form vapor and the vapor is caused to enter the condenser portion, in which it is condensed to provide purified alkylene carbonate in its liquid form. The liquid state alkylene carbonate is collected in the receiver portion and is recovered from the receiver.